Technical Field
Embodiments of the present invention relate to a light deflector, and devices such as optical scanner, image forming apparatus, image projector, heads-up display, and a laser radar into each of which the light deflector is incorporated.
Background Art
Light deflectors created through micro-electromechanical systems (MEMS) processing using silicon materials are known in the art. In such light deflectors, a mirror unit, a pair of torsion bars, and drive bars are formed as a single integrated unit. The mirror unit has a light reflection plane, and is supported by the pair of torsion bars. Each of the drive bars has a piezoelectric member and its one end is attached to a base. The other ends of the torsion bars that are not attached to the mirror unit are supported by the free ends of the drive bars. The mirror unit enters a moment of rotation where the mirror unit rotates when the bending deformation caused to the drive bars by application of voltage is transmitted to the mirror unit through the torsion bars. When the mirror unit rotates, the torsion bars twist and deform, creating an elastic force that restores the mirror to its original state. As this series of processes is repeated, the mirror unit rotates and oscillates at a constant amplitude.
For example, a configuration is known in the art in which the center of the mirror unit is displaced towards the connecting part of the drive bars and the base with reference to the axis of the torsion bars. Due to such a configuration, the moment generated for the mirror unit by bending deformation of the drive bars increases, and the rotation amplitude (deflection angle) of the mirror unit can be increased. In this known configuration, the center of rotation of the mirror unit is placed at the approximate center of gravity of the mirror unit.
In order to increase diameter of the mirror unit, the angle of the amplitude of the mirror unit, or the resonance frequency, the amount of displacement of the torsion bars (such an amount of displacement may be referred to as an offset amount in the following description) needs to be increased with reference to the center of the mirror unit so as to increase the moment. However, when the offset amount is increased, the asymmetry of the torsion bars increases with reference to the mirror unit. As known in the art, the mirror unit is designed to rotate around the center of the mirror. For this reason, the axis of the torsion bars is offset from the center of rotation of the mirror unit when the offset amount is increased, and extra deformation force (e.g., bending force) is applied to the torsion bars in addition to the torsion around the axis of the torsion bars.
In other words, when the drive frequency of the mirror unit becomes as high as a few kHz to a few tens of kHz, the mirror unit, which is made of silicon material, may deform due to the undesired deforming force caused by the offset amount of the torsion bars, and there are some cases in which the optically-required precision of the plane of the light reflection plane cannot be maintained. If the precision of the plane of the mirror unit is degraded, the beam spot that is formed by the beam reflected at the light reflection plane deforms, and the resolution deteriorates, leading to a low image quality.